1. Field of the Invention
The invention relates to a process for the preparation of vinyl ester polymer dispersions which are stabilized with protective colloids and have a high solids content, to the so-prepared dispersions, and to the use of these dispersions, or of the dispersible dispersion powders obtainable therefrom by spray drying, for example, for modifying building materials.
2. Description of Related Art
For economic reasons, dispersions having the highest possible solids content should be prepared. However, this requirement has not been met to date for all types of dispersion. Thus, dispersions which are based on vinyl ester/(meth)acrylate and are stabilized with protective colloids tend toward dilatancy at high solids contents.
The use of cationic azo initiators in the preparation of emulsifier-stabilized dispersions is known in principle. DE-A 32 39 091 describes a process for the preparation of cationic styrene or acrylic ester latices in the presence of cationic comonomers using a cationic azo compound as an initiator. The solids content of the dispersions described in the examples is a maximum of 41% by weight.
U.S. Pat. No. 4,489,192 relates to curable cationic polymer lattices which are based on vinyl esters and monomers containing cationic quaternary ammonium groups and are prepared in the presence of a water-soluble cationic azo initiator, an aminothiol salt, and cationic and/or nonionic emulsifiers.
A publication by K. Sakota and T. Okaya (J. Appl. Polym. Sci. 20, 1725 (1976)) reports on the preparation of cationic polystyrene latices, 2,2'-azobis(2-amidino-propane) hydrochloride being employed as an initiator. Stable latices are obtained only if the cationic emulsifier dodecylpyridinium chloride is added or in the case of copolymerization with the cationic monomer dimethyl-aminoethyl methacrylate hydrochloride. The solids content of the dispersions described is less than 25% by weight.